High frequency by-pass arrangement



Nov. 4, 1941. w. H. c. HIGGINS HIGH FREQUENCY BY-PASS ARRANGEMENT Filed July 26, 1939 H I Q F a 2 vv l l v R v NR 8 -ii H 2 Nu v IN VENTZ; W. H. C. HIGGINS BY AT TORNEV Patented Nov. 4, 1941 2,261,879 HIGH FREQUENCY'BY-PASS ARRANGE- MENT William H. C. Higgins, West Orange, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N..;: Y., a corporation of New York Application July 26, 1939, Serial No. 286,682

7 Claims. (01. 250-17) The invention relates to ultra-high frequency radio transmitters and more particularly to arrangements for adjusting, tuning and modulating such transmitters.

A feature of the invention is a tuning arrangement comprising a pair of Lecher wires of fixed length and. means for Varying the impedance between the said wires over a portion of their length, the proportionate part so modified being made variable for the purpose of tuning the system to a desired frequency. This feature is claimed in my copending application, Serial No. 412,940, filed September 30, 1941.

The feature of the invention claimed herein is a high frequency by-pass arrangement in conjunction with a motor-driven variable condenser, or the like, for by-passing high frequency currents which would otherwise enter the driving motor, where they might encounter a variable impedance, as in the bearings, for example, and cause interference with the normal operation of the high frequency system.

In the accompanying drawing:

Fig. 1 shows a frequency-modulated ultra-high frequency radio transmitter embodying the invention, the representation being partly schematic and partly in detail;

Fig. 2 shows an elevation of a mechanical arrangement for adjusting the coupling between a Lecher wire system and a coaxial line feeding an antenna, which arrangement appears in partial plan view in Fig. 1; and

Fig. 3 shows a cross-section of a by-pass condenser viewed as indicated by arrows 33 in Fig. l.

The radio transmitter of Fig. 1, which is illus-' trative of a system embodying the invention, is particularly adapted for use in a radiant energy distance indicating system. The transmitter generates an ultra-high frequency carrier wave which for the purposes of the distance indicating system is frequency modulated by means of a motordriven variable condenser which cyclically varies the frequency over a fixed range of values at a uniform rate. A distance indicating system employing a transmitter of this type is disclosed and claimed in a copending application of R. C. Newhouse, Serial No. 240,739, filed November 16, 1938.

The transmitter contains a vacuum tube I having a cathode ll, an anode I2 and a grid 13 and mounted between two pairs of Lecher wires (4, l5 and l6, 11, respectively, all preferably enclosed in a shield or metallic box I8. The tube an oscillator of a conventional type. A coupling loop [9 is provided within the box for taking electrical output from the oscillator and supplying it by means of a coaxial line 20 to an antenna or doublet 2|. One end of the loop I9 is connected to the inner conductor of line 20 and one branch of doublet 2| while the other end of the loop is connected to the inner conductor of another coaxial line 22, the length of which may be adjusted for tuning purposes by moving a shortcircuiting plug 23 with the aid of a knob 24. The outer conductor of line 20 is connected to the remaining branch of doublet 2|. Preferably the loop 19 is short compared with the length of the wires I4, l5, and is located close to the shield l8 near the ends of wires [4 and [5 where the current intensity in these Wires is a maximum. The connections to the lines 20 and 22 from the loop ends are preferably short and they extend through apertures in the shield l8. The outer conductors of the lines 20 and 22 are connected to the shield.

The coupling loop I9 is preferably a U-shaped piece of flexible metal and may be attached to the shield l8 by fixed, insulated supports at the two ends. Referring to Figs. 1 and 2, a screw 68 is attached by means of an insulating link 59 to the central portion of loop [9 in any suitable manner. A thumb-nut 10 fits into a hole in shield I8 and engages screw 68 to provide a means for deforming the loop by raising or lowering the central portion so that the coupling may be adjusted from the outside of the shield. Many other mechanical linkages may be used to provide remote control of the loop l9 and a suitable arrangement will be readily supplied by one skilled in the art to meet requirements in any particular case.

Provision is made for frequency modulation of the transmitter by means of a variable condenser 25 connected between the Lecher conductors l4 and IS. The condenser is rotated at a uniform rate by a motor 26 which is in turn driven by a generator 21. The motor is preferably a split phase synchronous motor with two windings which are supplied with currents in substantially quarter phase relation by means including a phasing condenser 28.

The oscillator may be tuned over a range of operating frequencies by a tuning mechanism comprising a metallic rod or conductor 30 mounted between the Lecher wires 16 and IT and extending through an aperture in the shield I8. The length of rod projecting within the shield and Wires constitute the principal components of may be adjusted by sliding the rod 30 in or out with the aid of a knob 3|. The rod 30 may be locked in any desired position by means of a clamp 32 and a clamping screw 33. The rod 30 is preferably conductively connected to the shield |8 through the clamp 32 or by a snug fit in the aperture through which the rod passes, but may, if desired, be insulated from the shield. The filament supply current for tube is introduced through a coaxial line comprising an outer conductor 34 and an inner conductor 35 connected respectively to the terminals of a battery 36 or other suitable source of current. The length of the coaxial line 34, 35 is preferably somewhat less than a quarter wave-length of the mean operating frequency of the transmitter.

The plate current for tube I0 is supplied by a battery 44 or other suitable source through a lead 45 and an insulated bushing to an enlargement 48 on the Lecher wire I6. A grid leak path is provided by a connection from an enlargement 50 on Lecher wire I! through a grid leak resistor 46 having an adjustable portion 41. The enlargement 48 is in the form of a metallic plate which is insulated from shield l8 by a sheet 49 of mica or other insulation. The plate 48, insulator 49 and shield |8 together form a condenser which effectively connects wire |6 to ground at frequencies in the operating range. The similar plate 50 and an insulator from another condenser together with shield l8 for grounding the wire Similar by-pass condensers are preferably provided between the shield l8 and the ends of the Lecher wires I4 and I5 remote from the tube I0. One end of the coaxial conductor 34 is likewise provided with a plate 52 and an insulator 53 forming together with shield I8 a condenser for grounding the outer conductor of the line at the operating frequencies. The insulator 53 is particularly useful when an alternating current source is substituted for battery 36 in which case it would be undesirable to have a direct current ground connection to the outer conductor 34. When a direct current source is employed, the insulator 53 may usually be omitted and the outer conductor 34 directly connected to the shield I8. Inner conductor 35 is similarly connected to the shield |8 by a by-pass condenser comprising a plate (I attached to the conductor, and an insulator 12. The elements ll and 12 are mounted on the outside surface of shield l8 and the latter forms one plate of the condenser as in the other by-pass condensers described above.

To permit tuning the cathode-ground circuit,

a condenser is arranged having a fixed plate 31 connected to outer conductor 34 at the tube end, and a movable plate 38. A shaft attached to plate 38 is threaded through a bearing 39 and is rotatable by means of an insulating rod 40 having a screw-driver slot 4| in one end, pro-- jecting from an aperture in shield |8. Rotation of shaft 40 controls the spacing between plates 31 and 38, thereby varying the capacity between them. This variable capacity may be adjusted to resonate with the inductive reactance existing I between the filament end of conductor 34 and shield l8. Due to the close coupling between the conductors 34- and 35, the inductive reactance between the conductor 35 and the shield I6 is also resonated at the same time. Where this coupling is insufficient a by-pass condenser may be added between the filament end of conductor 35 and the filament end of conductor 34. To provide suflicient friction to retain the movable plate 38 in any desired position, a section 42 of fibre is inserted into the the bearing 39. The plate 38 is grounded through the bearing 39 and a mounting bracket 43 which is fastened to the shield |8.

The rotor of the variable condenser 25 is coupled to the shaft 55 of the motor 26 through an insulating section 54. The motor is generally provided with ball bearings 56. It has been found by experience that high frequency currents from the Lecher wires l4, I5 pass through the capacity path comprising the metallic portion of condenser 25, the insulating shaft 54 and the shaft 55 of the motor 25 and through the ball bearings 56 to ground. Fluctuations in the output of the oscillator are encountered due to variations in the impedance of the path including the ball bearings or other moving portions of the motor. Accordingly a high frequency by-pass is provided comprising a condenser having one plate consisting of a metallic band 51 attached to the lower end of shaft 54 and a second plate comprising a metal collar or shield 58 surrounding band 51 but separated therefrom by a small air space. The shield 58 is connected to the shield l8 by means of a flange 59. The shield may be made in two parts for convenience in assembly.

Provision is made for determining the frequency generated by the oscillator. For this purpose a connection 60 is made to the outer conductor 34 of the coaxial line at a point selected to give a desired small voltage. The connection is extended by a lead 6| through a terminating resistor 13 and a coaxial line 62 to a wave meter 63.

To adjust the oscillator for a desired operating frequency, the position of rod 30 is varied until the desired frequency is obtained as shown by the wave meter 63. Moving the rod further into the interior of the shield raises the operating frequency and withdrawing the rod lowers the frequency. The rod may be locked in adjusted position by means of the clamp 32 and screw 33. The spacing between the condenser plates 31 and 38 may then be adjusted by means of a screw-driver in slot 4| to secure a maximum output from the transmitter consistent with the rating of the tube Ill. The antenna circuit may then be tuned to the operating frequency by sliding plug 23 with the aid of knob 24, preferably with loose coupling, and the coupling may thereafter be adjusted to any desired operating value.

The adjustment of the impedance of the Lecher wires l6, l! is made without changing the effective length of these wires and in this respect differs from the usual practice. The wires l6 and H are fixed in length and the inductive reactance is varied by varying the amount of the rod 33 extending inwardly through the shield l8. The presence of the rod 30 controls the characteristic impedance of the adjacent section of the Lecher system and the value of this impedance in turn controls the effective impedance of the Lecher system. The wires I4, I 5 are also fixed in length and are somewhat less than a quarter wave-length of the average operating frequency for which the oscillator is designed, but it has been found unnecessary to adjust the tuning of wires l4, l5, adjusting the rod 30 being all that is usually required. However, in general it is desirable that the complete Lecher system |4, l5, l6, I! shall be effectively a half wave-length long with the tube H! in the elec trical center.

When the motor 26 is operated, the variable condenser 25 is rotated at a constant rate to produce a continual frequency modulation of the transmitter. High frequency currents which would tend to flow into the motor and to ground through the ball bearings 56 are effectively bypassed by the condenser 51, 58 so as to eliminate substantially all fluctuations of oscillator output due to the variable impedance of the bearings or other portions of the motor. The construction of the by-pass condenser is shown in cross-sectional view in Fig. 3.

It will readily occur to those skilled in the art of ultra-frequency signaling that the several features of the invention are applicable to many systems other than the transmitter herein illustrated and in various other situations where the functions involved may be utilized.

What is claimed is:

1. In a circuit varying device having rotor and stator portions, a collector ring on the rotor portion, and a shield connected to the stator portion in spaced relation to said collector ring to form a condenser as a high frequency connection between the rotor and stator portions.

2. In a circuit varying device having a rotor and a stator, a by-pass condenser comprising one plate carried by the rotor and another plate attached to the stator, said plates being cylindrical in shape and mounted coaxially to provide an invariable capacitance therebetween during relative motion of said rotor and stator.

3. In a circuit varying device having a rotor and a stator, a by-pass condenser comprising one plate carried by the rotor and another plate attached to the stator in spaced relationship to said first plate, at least one of said plates being in the form of a surface of revolution coaxial with the rotor, every cross-section of which surface taken perpendicular to the axis of the rotor is substantially a full and complete circle, whereby the capacitance between the plates is rendered substantially invariant to relative motion of said rotor and stator.

4. In a circuit varying device having a rotor and a stator, a by-pass condenser comprising one plate in the form of a surface of revolution coaxial with the rotor and another plate in spaced relationship to said first plate, one of said plates being carried by the rotor and the other being attached to the stator, the capacitance between the plates being substantially unaffected by relative motion of the rotor and stator.

5. In combination with a circuit varying device having a rotor and a stator, a by-pass condenser having one plate comprising a cylindrical sleeve coaxial with and secured to the rotor and another plate comprising a cylindrical collar coaxial With the rotor, said collar being secured to the stator and juxtaposed to, but not in contact with, said sleeve.

6. In combination with a circuit varying device having a rotor: a driving motor coupled to said rotor through an insulating shaft; a high frequency electrical system connected with said circuit varying device; said circuit varying device, said insulating shaft and said motor forming a high frequency path to ground of inconstant impedance due to a variable contact within the motor; a cylindrical sleeve of conductive material secured to the said insulating shaft and coaxial therewith; and a stationary, grounded collar of conductive material in spaced relation to said cylindrical sleeve.

7. In combination with a circuit varying device having a rotor: a driving motor coupled to said rotor through an insulating shaft; a high frequency electrical system connected with said circuit varying device; said circuit varying device, said insulating shaft and said motor forming a high frequency path to ground of inconstant impedance due to a variable contact within the motor; a cylindrical sleeve of conductive material secured to the said insulating shaft and coaxial therewith; and a stationary, grounded, cylindrical collar of conductive material in spaced relation to and coaxial with said cylindrical sleeve.

WILLIAM H. C. HIGGINS. 

